Complex mesh deform with array for baking (necklace around the neck)
I'm not sure there's any easy way, except using the fantastic Wrapping Tools addon by Rich Colburn (which by the way allows you to create an unwrap version of any object).
You could do it with a cylinder used by the necklace as a Surface Deform object that would shrinkwrap around the neck, but when the shape begins to be complicated, this addon seems much more convenient.

Once you've installed the addon, duplicate the neck. Select an edge as if you were going to mark seam, and rip it (V).



In the N panel &gt; External Tools tab, press Create UV Shape. It creates a new shapekey for the object.



The panel is automatically refreshed, a Shape Control appears which acts like the Shapekey value. Tweak it to 1, the object flattens like a UV unwrap.



Select your necklace, put it just over the neck object, in Edit mode make it follow the shape of the neck.



Back to Object mode, select the necklace, shift select the neck, press Bind to Surface, then Scene Update.



Move the Shape Control value back to 0, the necklace is now around the neck. For the necklace as well it has created a second shapekey, you can delete the basis shapekey if you want.